It is known that dielectric films are used on a large scale for insulation purposes in applications such as transformers, generators and motors. A common problem associated with electrical apparatus so-insulated is that in high voltage applications, an electrostatic potential gradient along the surface of the insulator is created. This electrostatic potential gradient in the presence of air or any other gas will cause ionization of the gas particles which is called the corona effect. The corona effect causes the breakdown of the surface of the electrical insulation and will eventually totally destroy the insulating material. This problem is especially acute in high voltage applications such as generators and motors where a very large potential gradient is created.
As noted in U.S. Pat. No. 3,066,180 to Virsberg et al. many efforts have been made in attempting to obviate this problem. One method was to coat the surface of the insulation with a conductor having a suitable resistance. However, this method is rather ineffective in preventing the corona effect from appearing. Another method of obviating this problem is disclosed in the above cited patent and includes coating the insulator with a conductive component such as silicon carbide, which will further lessen the corona effect. However, the conductive coating is to be applied in either a thermosetting or air drying varnish which must then be placed on the insulator and then finally cured.
It is clear however, that certain problems are inherent with this method, in that many cumbersome and costly steps must be taken to apply and cure the varnish coating. In addition, the varnish coating substantially adds to the thickness of the dielectric film. It is desirable that insulators made from dielectric films be produced in uniform, thin sheets with simple techniques similar to those used in standard paper making.
It is known, for example, to heat mica to loosen inter-laminar bonds, then to slurry it in water, and finally to form it into a mat on a paper making machine. The product of this procedure is then wound into rolls for later use as sheet insulation or as tapes and the like. Such a dielectric is rather fragile, and resins, such as silicones, polyesters, epoxys, polyimides, polyolefins and the like, can be added to fill voids in the dielectric. Alternatively, or in addition, thinner resin films, e.g., poly(ethylene terephthalate) films can be used to strengthen the dielectric films, and/or there can be added layers of glass cloth or undirectional glass filament mats to strengthen such sheets. In any case, it is further desirable that the insulator be as thin as possible so that when used in electrical devices the tape or sheet insulator will occupy a small area, thereby allowing for an increase in the size of the current carrying components. This arrangement will permit an increased output from electromagnetic components having a similar overall size, thereby lowering the manufacturing cost while improving operating performance.
It is noted that the use of aluminum or aluminum oxide coatings is found in the prior art. As an example, U.S. Pat. Nos. 2,748,019 to Schramm, Jr., and 2,156,083 to Dalton, aluminum is used in textile fabrics for heat insulation and in a moisture protected gum coating respectively. In U.S. Pat. Nos. 3,935,375 to Ichiba, et al, and 3,514,326 to Stow, aluminum is used as a conductor in laminated sheathed cable and a conductive tape. In U.S. Pat. No. 3,780,206 to Reynolds, aluminum oxide is provided in fibrous sheet material to be used as a conductor or a semi-conductor.
As noted above, efforts in the prior art to produce dielectric films with enhanced resistance to high voltage breakdowns resulted in procedures that were complex, time consuming, and costly. In addition, the resulting tapes had an increased thickness due to the varnish coatings. Efforts to reduce the thickness of the insulators would only result in a corresponding decrease in the structural integrity of the tape. Therefore, it would be desirable to more easily produce a dielectric tape having properties which inhibit the corona effect and decrease the likelihood of breakdown. Further, it is desirable that the dielectric tapes be of minimal thickness and yet still be capable of being utilized in high voltage applications.
It is therefore an object of the subject invention to provide a dielectric film with an increased voltage endurance that can be manufactured at low costs. It is a further object of the subject invention to provide a dielectric film with minimum thickness. Still another object is to provide a mica-based dielectric tape strengthened with resin and/or glass fiber reinforcement having increased voltage endurance.